Method and apparatus for the continuous heat-treatment of metal strip



June 3%, i959 MYERS 2,892,744-

METHOD AND APPARATUS FOR THE CONTINUOUS HEAT-TREATMENT OF METAL STRIP Filed July 23, 1956 2 Sheets-Sheet 1 Q a: 9 M

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United States Patent METHOD AND APPARATUS FOR THE CONTINU- OUS HEAT-TREATMENT OFMETAL STRIP Henry F. Myers, El Cerrito, Calif., assignor to United States Steel Corporation, a corporation of New Jersey Application July 23, 1956, Serial No. 599,640

8 Claims. (Cl. 148-16) This invention relates to a method and apparatus for heat-treating metal strip continuously in single-ply or "strand form. 'More particularly, it concerns the annealing of ferrous strip, e.g., cold-reduced steel strip.- The term strip, as used herein, is intended to include wire and rod. 1

Steel strip is now being annealed in substantial tonnage by drawing it continuously through a tower-type furnace. Such installations are very expensive, however, and are justified only if a high production can be maintained. One limit to 'the production attainable from equipment of a given size is the rate at which heat can be imparted to the strip and removed therefrom. Present equipment requiresfrom 25 to 40 seconds to raise the temperature of a point on the traveling strip to from 1250 to 1350 and give the desired soak or time at temperature. It is accordingly the object of my invention to provide a method and apparatus capable of heating the strip at a rate much higher than can be achieved by known practice and apparatus. I

In a preferred embodiment and practice of the invention, I pass the strip through a preheating chamber in which it is immersed in a bath of molten metal and then through a final heating chamber wherein it is subjected to contactwith the vapor of said metal. The strip then passes through a. quenching bath of the aforesaid molten metal and then through a holding or soaking chamber. Finally the strip passes through a second quenching bath of said molten metal. A non-oxidizing atmosphere is maintained in the holding chamber and above the surface of the molten metal in the preheating and quenching chambers.

Hot metal vapor is delivered to the final heating chamber from a suitable boiler. Liquid metal resulting from condensation of the vapor in the heating chamber collects in the preheating chamber whence a portion is circulated successively through the tanks holding the quenching baths while the remainder is returned to the boiler. As a heating medium, the metal sodium has many advantages. Other alkali or alkaline-earth metals or alloys thereof may also be used for the purpose.

A complete understanding of the invention may be obtained from the following detailed description and explanation which refer to the accompanying drawings illustrating the present preferred embodiment.

In the drawings:

Figure 1 is a'central longitudinal section through the apparatus; and

Figure 2 is a schematic diagram showing the travel of the strip through the several steps of the process and the circulation of metal vapor and condensate from the boiler, through the apparatus and back to the boiler.

Referring now in detail to the drawings and, for the present, generally to Figure 1, the apparatus there shown comprises a preheating chamber 10, a final heating chamber 11, a first quenching tank 12, a holding or soaking chamber 13 and a second quenching tank 14. Chamber includes insulated wells 10a, 10b and 100 in cascade arrangement, for holding molten metal. Ferrous strip 15 is drawn through the several chambers and tanks successively, being trained in a plurality of vertical passes over vertically spaced pairs of guide rollers. These rollers have shafts extending through the walls of the chambers and tanks, sealed thereto and journaled in suitable external bearings. The upper and lower rollers in the preheating chamber are designated 16 and 17, respectively, those in the final heating chamber, 18 and 19; and those in the holding chamber, 20 and 21. Guide rollers 22 are mounted in the connection between chamber 11 and tank 12. The tanks 12 and 14 are provided with bottom rollers 23.

The chambers 10, 11 and 13 and tanks 12 and 14 are constructed from metal plate of suitable composition, e.g., stainless steel, and are provided with thermal insulation as needed to limit loss of heat. Vertical baflles of metal plate are mounted in the several chambers and tanks, designated as follows: in the preheating chamber, 24; in the final heating chamber, 25; in the holding chamber, 26; and in the quench tanks, 27 and 28. The baflies define separate compartments for the passes or groups of passes of the strip upwardly and downwardly, and control the flow of heating medium, or serve to separate one chamber from those adjacent thereto.

Sodium vapor from a suitable boiler B (see Figure 2) is delivered to the top of chamber 11 through ports 29. The vapor coming in contact with the traveling strip gives up some of its heat thereto and condenses. The condensate collects in sealing pools in tanks 3% at the bottom of chamber 11, in which rollers 19 are located, overflows therefrom into the preheating chamber 10 and flows by gravity through the several wells 10a, 10b and 100 thereof, which are at successively lower levels as shown. A portion of the condensate is returned to the boiler through a pipe 31 by feed pump 32. The remainder of the condensate is delivered to tank 14 through pipe 33 by pump 34. The condensate in tank 14 is continuously recirculated by a pump 35 and connections 36 and 37. A heatexchanger 37a on connection 37 permits control of the temperature of the condensate in tank 14. At the same time a portion of the condensate is taken oif by pump 38 and delivered by a connection 39 to tank 12. From tank 12, condensate is drawn through a pipe '40 and a heat-exchanger 41 by a pump 42 and returned to well 10m.

A neutral or non-oxidizing atmosphere such as nitrogen is maintained in chamber 10 above the surface of the condensate and in chamber 13, the gas being delivered thereto from a suitable source (not shown) through inlets 43. Chamber 10 has entry seal rollers 44. An atmospheric condenser 45 is connected to chamber 10 to receive excess sodium vapor from chamber 11 during a stoppage of the strip, until the supply of vapor from the boiler can be reduced. The top outlet from the condenset has a check valve 45a therein. A side connection to the outlet provides an emergency inlet for protective atmosphere and has a similar valve 45b therein. A partition 47 in chamber 10 having a slot therein for the strip, tends to confine any sodium vapor escaping from chamber 11 so it will enter the condenser 45. Heating chamber 11 is provided with a vent 48 for the removal of non-condensable gases evolving from the strip as it is heated.

Holding chamber 13 has a recirculating duct 49 connected thereto at spaced points and provided with a blower 50. Heat-exchanger 41 is located in duct 49 whereby the atmosphere of the holding chamber, on being recirculated by the blower, is reheated to maintain the proper tem- Operation of the apparatus of Figure l for perforate:

ance of the method of my invention will now be explained, with further reference to Figure 2. Steel strip 15, e.g., cold-rolled, low-carbon strip, entering between the seal rollers 44 is preheated from, say 100 F. to about 1050" F. by several successive passes'through molten sodium in wells 10a, 10b and 100. The strip then enters chamber 11 and is there heated by direct contact with sodium vapor to a temperature of about 1618 F. (B.P. of sodium=16l8.7 F.i8.5). On emerging from the exit seal tank 30, the strip enters quench tank 12, where its temperature is reduced to about 1100 F. The temperature of the strip remains substantially unchanged while passing through chamber 13, resulting in isothermal transformation. On leaving chamber 13, the strip is quenched in tank 14 to about 600 F. and, after passing through exit seal rolls 53, is water quenched in a tank 54 to about 200 F., rinsed in a spray chamber 55 and dried in a dryer 56. I

Figure 2 also shows the circulation of the heating medium. Sodium vapor leaving the boiler at about 165 F. condenses into liquid sodium at about 1618 F. in chamber 11. It is further cooled to about 575 F. by the time it leaves chamber 19. The portion of condensate returning to the boiler is first cooled to about 300 F. in a heat-exchanger 57, filtered in a filter 58 and collected in a storage tank 59. The remainder passes first through quench tank 14 and then tank 12, being heated by the strip entering the latter to about 1080 F. before returning to chamber 10. Heat-exchanger 37a removes heat from the recirculating portion of the molten sodium in tank 14, to prevent the temperature thereof from rising to an excessive value.

The process described above is but one example of a number in which the principle of contact of the work with liquid and vaporized metal may be used for heattreating. The described method includes heating ferrous strip to its austenitizing temperature by heat transfer in chambers and 11, quenching to an isothermal annealing temperature in quench tank 12, holding at isothermal annealing temperature in chamber 13 and quenching to a lower temperature in second quench tank 14 which prepares the strip for further processing or for exit into the air without formation of surface oxide to an objectionable degree.

An alternative would include heating as above, then quenching to a temperature in first quench tank 12 to cause transformation to martensite, holding at this temperature or cooling further in the holding chamber, then raising the strip temperature in the second quench tank 14 to produce a tempering effect. A still further modification might include either of the foregoing with delivery of the strip from exit seal rolls 53 into a protective atmosphere in a subsequent process or into a bath of molten coating metal such as zinc, aluminum, tin or the alloy used to make terne plate. By this, advantage is taken of two properties of my process, viz., its heat-treating action and its ability to prepare surfaces for subsequent coating operations by reason of the high reactivity and deoxidizing power of the liquid alkali metal and vapor. Other modifications will suggest themselves to those skilled in the art.

As will be evident from the foregoing, metal vapor from a suitable boiler is a primary source of heat input to the process. Heat transfer to the strip in chamber 11 is effected by several methods, viz., radiation, convection, conduction and condensation of metal vapor on the strip surfaces. Except for the slowest speeds, the condensation factor predominates. In the preheating baths 30a, 10b and 100, bafiies 24 function to maintain nearly counter-current flow of the metal with respect to the strip and hence better heat recovery from the liquid metal leaving the quench tanks 12 and 14 or the seal tanks 30. Bafiles 25 provide for sealing the metal vapor from the balance of the process under normal operation and permit a difierence in pressure to exist between chamber 11 and chambers 10 and 13 while at the same time together with the liquid metal, preventing excessive pressure rise or fall of the heating vapor in chamber 11. That is, excess pressure will be relieved by displacement of liquid metal.

In the event metal vapor is supplied faster than required by the process, the pressure will build up to the point where vapor will escape into the preheating cham her and find its way to the condenser 45 where is will be condensed. The resulting liquid metal can be returned to the preheating chamber 10 for heat recovery or to a storage tank. Baffle 47 with restricted strip clearance hinders fiow of excess metal vapor into the preheating chamber and aids its confinement to the condenser 45. In the event of a very rapid rise in pressure, the check valve 45a will permit the uncondensable portion of the vapor to exhaust to a stack or vent. If the metal vapor is being supplied at too low a rate for the process, a pressure decrease in the heating atmosphere will cause a change in metal level in sealing tank 30 which will permit protective atmosphere to flow in through check valve 45b and into chamber 11, diluting the metal vapor until its efiect on reducing the rate of heat transfer, is such that the rate of condensation of metal vapor is substantially equal to the rate at which it is being supplied. Baffles 26 in holding chamber 13 serve to prevent short-circuiting of the gas flow. Baffle 25 acts similarly in chamber 11. Heat-exchanger 37a permits removal of heat from or its addition to the liquid metal flowing into quench tank 14. Vent 48 permits continuous removal of a portion of heating vapor to a condenser, whereby to remove some of the non-condensable gas evolved from the strip and keep the heat transfer rate between heating vapor and the strip at a maximum.

The prime advantage of the invention is the fact that, because of the highly efiicient transfer of heat from the liquid metal and vapor to the work, the temperature of the latter may be raised to from 1600 to 1625 F. in five seconds, including a holding period or time at temperature of two seconds. At this heating rate, a high production may be obtained from a small installation because of the high speed of travel which is permissible. That is to say, since the total length of the path in the apparatus is the product of the processing time and speed of travel, the shorter the time, the smaller the apparatus at a given speed or the higher the production for a given size of apparatus.

There are several advantages obtained by the use of sodium as a heating medium. The extremely high latent heat of vaporization of sodium of 1868 B.t.u. per pound and its boiling point of 1618 F. makes it a self-regulating heating medium at a very appropriate temperature. This combination is unique in metals that do not alloy with iron. A film of liquid sodium has very high heattransfer coefiicients in contact with steel. They are considerably greater at the same relative velocity than the coeflicients of other liquid heat-transfer baths, such as lead, lead-bismuth, high temperature salts or Dowtherm. Any contamination of the strip with sodium is totally removed in the simple water-quench operation. Sodium vapor provides simultaneously a reducing and a non-decarburizing environment. The low price, low density and ready availability of sodium makes it the most economical of metals for heat-transfer purposes. Sodium-potassiurn alloy (99-20% Na, 180% K) possesses many similar characteristics at lower boiling points, de pending upon the composition. In liquid baths, however, pure sodium is preferred because of its higher heat conductivity. I

In the case of heat-treatment of ferrous strip, therefore, the heating medium may be selected from the alkali metals or a mixture of them, and may include appreciable quantities of metals from the alkaline-earth group which react only slowly with ferrous metal at the heatn-eatment temperatures employed. Sodium is almost uniquely ideal for this process because its boiling point at atmospheric pressure is very close to the temperature for austenitizing many ferrous alloys. Use of a heating metal which is nearly pure yields the process its closest control over the maximum strip temperature. This maximum will be close to the boiling point of the heating metal at the pressure existing in the heating chamber. The effect of any moderate superheat in the incoming metal vapor will be negligible, except at the time of sudden reductions in processing speed or when, for other reasons, the rate of supplying metal vapor may greatly exceed its rate of condensation in the heating chamber 11.

A further advantage is the self-regulating characteristic of the heating system. If the strip traveling through the apparatus has to be stopped suddenly in an emergency, its temperature does not build up as it would in the case of high-gradient radiant heating, induction or resistance heating, possibly causing burn-through of the strip, since the maximum temperature of the vapor is only a few degrees above the boiling point of the metal and the cessation of heat absorption by traveling strip merely means that less vapor will be condensed. Provision is made as already explained for disposing of excess vapor during strip stoppage, until the boiler output can be reduced. At the same time, the temperature of the heating chamber is maintained ready for restarting at any instant. The factors just touched on permit a material speed reduction during the welding of coil ends. Thus the amount of surplus strip necessary to be stored in the usual looper is less and the size and cost thereof are likewise reduced.

A still further advantage is the elimination of gradual cooling of the strip over a time of from 30 to 60 seconds. By quenching in two stages with isothermal transformation therebetween, the overall processing time is materially shortened and the cost of the apparatus reduced. The recovery of heat in the quenching baths also reduces the operating cost. The heat consumption, in fact, is only about three-fourths of that of conventional stripannealing lines per ton of product.

Although I have disclosed herein the preferred embodiment of my invention, I intend to cover as well any change or modification therein which may be made without departing from the spirit and scope of the invention.

I claim:

1. A method of annealing metal strip which consists in preheating the strip by bringing it in contact with a bath of molten alkali metal, said metal having a vaporization temperature in the neighborhood of the annealing temperature of said strip, then passing the strip through a heating chamber while supplying thereto from an external source the vapor of said metal, and conducting to said bath the liquid metal condensing in said chamber.

2. In a method as defined by claim 1, characterized by maintaining sealing pools of the liquid condensed from said metal vapor at the entrance and exit of said chamber and passing the strip through said pools successively as it enters and leaves said chamber.

3. Apparatus for continuously heat-treating metal strip comprising a conjoined preheating chamber, heating chamber and soaking chamber, means for guiding strip through said chambers successively, means including a boiler separate from said chambers for supplying to said heating chamber the vapor of a metal having a vaporization temperature in the neighborhood of the annealing temperature of said strip, means at the bottom of the heating chamber for containing a pool of liquid metal, collecting by condensation of said vapor, said preheating chamber communicating with said containing means and being so located relative thereto as to receive metal therefrom for initial contact with the entering strip, an initial quench tank between said heating chamber and said soaking chamber, a second quench tank at the exit end of said soaking chamber, means supplying liquid metal from said preheating chamber to said second quench tank and means supplying liquid metal from the second quench tank to the initial quench tank.

4. A method of annealing steel strip consisting in drawing the strip in single ply through a bath of molten alkali metal at a temperature of about 1000" F. drawing the strip from said bath through a heating zone, generating vapor of said metal apart from said bath, supplying the vapor to said zone at a temperature of about 1650 F., collecting the molten metal condensing in said heating zone into sealing pools at the entrance to and exit from said zone and draining metal from at least one of said pools into said bath.

5. A method as defined by claim 4, characterized by passing the strip through a quench tank after it leaves the heating chamber and supplying liquid metal to the quench tank from one of the sealing pools.

6. A method as defined by claim 4, characterized by passing the strip through a first quench tank after it leaves the heating chamber and through a second quench tank after it leaves the soaking chamber, and circulating liquid metal from one of the sealing pools through said quench tanks in reverse order.

7. A method as defined by claim 4, characterized by maintaining a non-oxidizing atmosphere in said soaking chamber, circulating said atmosphere through a heatexchanger and passing liquid metal from said bath through said exchanger.

8. A method as defined by claim 4, characterized by subdividing said bath into pools and drawing the strip successively through said subdivided pools and causing the liquid metal to cascade from one of said subdivided pools to the next.

References Cited in the file of this patent UNITED STATES PATENTS 843,563 Frith Feb. 5, 1907 1,987,577 Moers Ian. 8, 1935 2,231,009 Holt Feb. 11, 1941 2,418,088 Nachtman Mar. 25, 1947 2,441,500 Miess May 11, 1948 2,458,525 Nachtman Jan. 11, 1949 2,463,412 Nachtman Mar. 1, 1949 2,573,019 Hess Oct. 30, 1951 2,797,173 Keller June 25, 1957 2,797,177 Keller June 25, 1957 FOREIGN PATENTS 710,081 Great Britain June 9, 1954 

4. A METHOD OF ANNEALING STEEL STRIP CONSISTING IN DRAWING, THE STRIP IN SINGLE PLY THROUGH A BATH OF MOLTEN ALKALI METAL AT A TEMPERATURE 1000* F. DRAWING THE STRIP FROM BATH THROUGH A HEATING ZONE, GENERATING VAPOR TO SAID ZONE AT A TEMPERATURE OF ABOUT 1650* F., VAPOR TO SAID ZONE AT A TEMPERATURE OF ABOUT 1650* F., COLLECTING THE MOLTEN METAL CONDENSING IN SAID HEATING ZONE INTO SEALING POOLS AT THE ENTRANCE TO AND EXIT FROM SAID ZONE AND DRAINING METAL FROM AT LEAST ONE OF SAID POOLS INTO SAID BATH. 